Of the three xylene isomers, paraxylene is the most commercially valuable. However, due to the similarity of their boiling points, adsorption is a commonly used method to separate paraxylene from the other xylene isomers, in which an adsorbent solid that preferentially adsorbs paraxylene over metaxylene and orthoxylene is used. Simulated moving bed (SMB) separation is especially useful for separating paraxylene from a mixture of C8 aromatics.
A commercial embodiment of a simulated moving bed adsorption apparatus is used in the well-known Parex™ Process, which is used to separate C8 aromatic isomers and provide a more highly pure paraxylene (PX) stream from a less highly pure mixture. See by way of example U.S. Pat. Nos. 3,201,491; 3,761,533; and 4,029,717. Other embodiments involving a simulated moving bed adsorption apparatus include the commercially available ELUXYL® and AROMAX® processes.
In a Parex™ unit, the locations of liquid input and output are moved by a fluid directing device described herein as a rotary valve device. This device may comprise one or more rotary valves, as well as various control and accessory means, such as inlet lines, outlets lines and valves associated therewith. The rotary valve device works in conjunction with conduits located between the adsorbent beds. The rotary valve device accomplishes moving the input and output locations through first directing the liquid introduction or withdrawal lines to specific conduits in fluid communication with particular adsorbent beds. After a specified time period, called the step time herein, the rotary valve device advances one index and redirects the liquid inputs and outputs to the conduit immediately adjacent and downstream of the previously used conduits. Each advancement of the rotary valve device to a new position is generally called a valve step, and the completion of all the valve steps is called a valve cycle. The step time is uniform for each valve step in a valve cycle, and may be from about 30 seconds to 4 minutes.
An example of a simulated moving bed adsorption apparatus contains 24 adsorbent beds and 24 conduits individually connected to a bed and providing fluid communication with the rotary valve device. The conduits of the adsorption apparatus may function, over time, as at least two liquid input lines (e.g., a feed input line and a desorbent input line) and two liquid output lines (e.g., an extract withdrawal line and a reformate withdrawal line).
In some simulated moving bed systems, a second rotary valve is used in parallel to allow for extra capacity or improve continuity of operations. A system with two rotary valves is described in U.S. Pat. No. 8,168,845.
In standard simulated moving bed separation processes, the flow rate of streams into and out of the simulated moving bed are held constant during the step time. However, modulation of flow during the step time has been found to enhance separation in certain instances involving simulated moving bed separation of fructose and glucose or separation of 1,1′-bi-2-naphthol enantiomers. The enhanced separation may result in greater purity of product streams or less desorbent use. This process for modulating flow rates during a step time has been referred to as a PowerFeed process. Examples of PowerFeed processes are described in an article by Kawajiri et al., “Optimization strategies for simulated moving bed and PowerFeed processes”, AIChE J. Vol. 52 (2006) B, pp. 1343-1350, and in an article by Zhang et al., “PowerFeed operation of simulated moving bed units: changing flow-rates during the switching interval”, Journal of Chromatography A, Vol. 1006, pp. 87-99, 2003, Elsevier B.V.
International Application publication No. WO 2016/133589 discloses a PowerFeed process, in which the flow rate of feed to the simulated moving bed apparatus is varied during each interval to enhance the separation of paraxylene from the multicomponent mixture. A challenge of using a PowerFeed process is that it requires larger and higher capacity peripheral equipment, such as attendant distillation towers, pumps, surge vessels, and heat exchangers, in order to handle the cyclic flows and much higher peak flow rate of streams as compared with a non-PowerFeed process where the flow rates remain constant, increasing cost and energy consumption. In addition, a PowerFeed process may have a high frequency of flow variation, which can accelerate wear and tear of peripheral equipment.
Thus, there is a need to further improve the PowerFeed mode of operation in a simulated moving bed adsorption process in a way that can make the process more efficient, minimize the size and/or complexity of peripheral equipment, and prolong the life cycle of peripheral equipment. Also, there is need for a PowerFeed process that can be easily implemented on existing facilities without significant modifications to the peripheral equipment, thereby minimizing capital investment.